The present invention is drawn to a process and apparatus for continuously monitoring and calculating the flow rate of a fluid medium in a flow line where a chart recorder is used as part of the monitoring process and more particularly, to a process and apparatus for continuously monitoring and interpreting the traces produced by a chart recorder to determine gas flow rate.
Chart recorders have been used for many years in numerous industries for recording the measurements of physical quantities, as in the extensive use by the petroleum and chemical industries for determining the flow rates of fluids, particularly gases, in order to determine consumption and to serve as a visible record and evidence of said consumption, particularly when the fluid or gas is a commercial product provided by a supplier to a consumer. The traditional method of determining flow is by computation from determinations of the gauge pressure of the gas and the differential pressure across an orifice plate inserted into the flow line. Since the flows are usually required to be measured continuously and often in remote areas, simple pre-calibrated automatic circular chart recorders are usually used to register the gauge and differential pressure components as traces on a removable chart. By measuring atmospheric pressure or determining its value from knowledge of the ambient temperature and the altitude at which the measurements are made, the absolute pressure of the gas can be derived. The flow is then determined from the product of the calibration constant of the orifice times the magnitude of the square root of the product of the absolute pressure and the differential pressure. For strict accuracy in calculations, the square root of the cross-product of the two dynamic pressure variables must be continuously integrated over time. For lack of simple, economic and automatic methods of performing the correct calculation, it has been common practice to independently integrate and thus average the two pressure variables over time and to then extract the square root of the product of these averages.
In the prior art, there are many different types of mechanical integrators for the postprocessing of charts including wheel and disc multipliers used by human operators and various types of cams for determining square roots. A number of different chart conditions can increase the degree of difficulty in obtaining an accurate chart reading. For example, some charts show a solid, wide band where the chart pen has oscillated back and forth rapidly relative to the slow rotation of the chart over day-long periods. The human chart interpreter often has to make a guess as to where the pen was located for the majority of the time, in effect applying an unwarranted averaging process. Another type of chart that is very difficult to read manually is that having a sun-burst trace where there are about one hundred strongly-curved excursions of very large amplitude. Such traces are extremely hard to retrace on a moving platform with any pantograph or curve-following mechanism.
Other devices are known that essentially modify the basic chart recorder itself to perform the calculations by acting as a planimeter or by electronically computing and recording the results on the chart. The problem with these more complicated devices is that one is required at each and every monitoring site. This can range from several score to many thousands of devices per user organization.
U.S. Pat. Nos. 1,877,810 and 2,611,812 exemplify gas flow recording devices which utilize variable inductances or movable core transformers responsive to fluid pressure to generate an electrical signal and another similar inductance device, usually at a remote location for a remote recorder, to reconvert the electrical signal to a mechanical movement so as to move a recording pen on a rotatable chart. Although these patents disclose a remote recording instrument, they fail to solve the problem of how to accurately read and/or interpret the chart and determine flow rate therefrom. In addition, these systems are cumbersome to use because additional equipment must be provided at each and every flow measurement location and at each and every recorder location. Further, appropriate signalling systems must be provided between each measurement device and each recorder.
Another prior art system is shown in U.S. Pat. No. 3,322,339 wherein the mechanical system is physically connected to the chart recorder and mechanically acts as a planimeter to calculate the area under the curves, thus eliminating the need for a manual operator to trace over the curve on the chart. This mechanical system places an extra load on the movement of the pen, which could cause inaccuracies due to the extra loading. Also, it has many moving parts, and lost motion at the pivot points could cause further inaccuracies.
U.S. Pat. No. 3,742,515 discloses an analyzer for a chart recorder which utilizes electrical signals to produce a triangular wave traced along the edge of the recording chart, with each excursion representing a particular amount of area of integration of the amplitude of the analog signal. This analyzer does not however obtain an electrical signal showing the total quantity of fluid flow, nor even rate of fluid flow. Further, there are no means for interpreting the resulting charts other than manually.
U.S. Pat. No. 3,980,865 provides an electronic integrator for use in conjunction with a mechanical apparatus for transcribing the traces of pressure and differential pressure on circular flow charts. The electronic integrator, by proper encoding of the rotation of the chart table and of the two angular motions of the two pen arms, provides a running integral of the flow. Operation of this device requires an operator to simultaneously guide two pointers over two traces on the rotating gas chart, using one mechanical control lever in each hand to control each pointer, while at the same time the integrator performs the desired computations. Since both hands are occupied, the operator cannot easily disengage operation if the traces are incomplete. The device also relies on the dexterity of the operator for accuracy. Other disadvantages associated with this device includes (1) operator difficulty in accurately and simultaneously tracking two traces on a rotating table; (2) operation only in accordance with predetermined operations selected by a control panel; and (3) slow driving speeds to enable the operator to process charts with high-frequency trace displacements.
Finally, U.S. Pat. No. 4,414,634 relates to a data totalizer for fluid flow which is electronically calculated and directly connected to a chart recorder. The data totalizer includes first and second linear variable displacement transformers, with linkage connected directly in the chart recorder to move in accordance with movements of a plurality of marker pens and connected to move the linearly displaceable elements in the two transformers. An electronic multiplier is provided to multiply together the electrical outputs of the first and second transformers to obtain a product. An electronic circuit is provided to calculate the square root of such product to obtain an electrical signal proportional to the rate of fluid flow. This signal may be multiplied by a time signal to obtain the total quantity of fluid flow.
While the foregoing U.S. patents represent improvements in the state of the art, there are a number of differences vis-a-vis the present invention. For example, the devices accomplish the improvement either by the addition of the described devices to existing chart recorders or by proposing an improved chart recorder of greater complexity thus increasing the quantity of equipment at each flow monitoring point without providing the ability to retain measurements other than graphically. In particular, they do not show how to continuously monitor chart recorders and accurately compute pressures and integrate flows over time.
Accordingly, it is a principal object of the present invention to provide a process and system for continuously monitoring and calculating fluid flow rate in a flow line.
It is a further object of the present invention to provide a process and system as above which is rapid, highly automated, and highly accurate.
It is a particular object of the invention to provide a process and system as set forth above which includes a video monitoring of a plurality of trace lines on a flow chart recorder.
It is yet a further object of the present invention to provide a process and system as set forth above which automatically interprets the trace lines on the flow chart recorder and determines therefrom the manometric pressure and differential pressure for calculating the fluid flow rate.
These and other objects and advantages will become more apparent from the following description and drawings.